A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In optical lithography, for example, the illumination source is of a relatively high energy or high level of flux in order to achieve a required exposure dose and the patterning device will absorb a significant fraction of this energy. Additionally or alternatively, the patterning device may receive energy from other sources, such as motors, etc., particularly in non-optical lithography. As a result, the patterning device will increase overall in temperature and this increase may be uneven and pattern dependent. Overlay and/or focus induced problems may occur and thus performance of the lithographic apparatus may therefore be adversely affected. The heating of the patterning device may be complicated in a scanning lithography system, where, for example, the surface of the patterning device scans across a high energy illumination field.